1. Field of the Invention
This invention relates to a rear casing arrangement for a magnetic drive pump, the magnetic drive pump comprising a front casing and a rear casing defining a pump chamber together with the front casing, the rear casing being rotatable in unison with an impeller in the pump chamber and intervening between a rotor having a driven magnet and drive magnet disposed outside thereof.
2. Description of the Prior Art
The rear casing is required to have corrosion resistance as required in the case of using chemical fluid or semiconductor processing fluid as pump supply fluid and also pressure-resistant mechanical strength to resist pressure applied to the pump supply fluid in the pump operation. In addition to the pressure-resistant mechanical strength, in the case of using high-temperature reactive fluid as the pump supply fluid, heat resistance is required. Among the rear casing structures are a single layer or single member structure, in which a single member copes with the above functions, and a so-called two-layer structure, which comprises an inner casing member disposed on the side of contact with the pump supply fluid for meeting the corrosion resistance, and a cover member, which is disposed on the outer side of the inner casing member for meeting the pressure resistance, and hence the mechanical strength. Either member of this two-layer structure is required to be heat-resistant in function.
The single member structure rear casing is formed from such materials as thermoplastic resins, which can be readily injection molded as well as being corrosion-resistant, and fragile or brittle materials typically ceramics. However, the thermoplastic resins are insufficient in the pressure-resistant and heat-resistant mechanical strength. The ceramics, on the other hand, have problems that they are weak against temperature changes and also lead to high cost of manufacture, although they provide high pressure-resistant mechanical strength. As an improvement against these drawbacks, the above two-layer structure has been proposed. In an example of this structure, a thermoplastic resin which is richly corrosion-resistant and capable of being readily molded, is used for the inner casing member, and a metal cover is used as the mechanical strength cover member. However, the metal cover leads to cost increase, and also has problems of efficiency reduction due to eddy current generated in the space between the drive and driven magnets and also attaching of the inner casing member by heat generation. To cope with these problems, it has been proposed to form the mechanical strength cover member from fiber-reinforced plastics, which are composite materials obtained by combining a fibrous reinforcing material for providing for the mechanical strength and thermoplastic resin. In this case, however, the mechanical strength is inevitably reduced by temperature increase of the thermoplastic resin. A further cover member proposal has a structure obtained by combining a thermoplastic resin and a fibrous reinforcing material. This structure has an advantage that it can hold relatively stable mechanical strength irrespective of temperature changes. However, it has a problem in its manufacture that the degree of shape freedom is low, thus leading to high cost of manufacture.
At any rate, the rear casing of any of the above kinds is interposed in a narrow space between the drive magnet and a rotor having a driven magnet. Therefore, a restriction is imposed on the material thickness, and it has thus been extremely difficult to obtain a shape which is advantageous in view of the mechanical strength.
The present invention has been made in view of the above various problems, and it has an object of providing a rear casing arrangement for a magnetic drive pump, which does not use any metal subject to eddy current generation which is undesired as material, but chooses a synthetic resin material, while solving the mechanical strength problems by adding a reinforcing ring member having a cylindrical shape which can be readily manufactured, so that it can sufficiently meet each function necessary as the rear casing and also be manufactured at low cost.